The present invention relates to filters used in radio communication circuits. More particularly it relates to filters that convert mechanical vibrations into an output signal at frequencies of MHz band or GHz band, where resonators having a size of xcexcm order are used. This micro-mechanical resonator is excited by an input signal having a frequency around the resonance frequency of the resonator, thereby producing the fine mechanical vibrations to be converted by this filter to the output signal.
A conventional filter is disclosed in IEEE Journal of solid-state circuits, Vol. 35 No. 4, April 2000, issue. FIG. 11 shows a structure of a conventional filter that is formed on substrate 90. This filter comprises input line 94, output line 95, two resonators 91, 92 of which both ends are fixed to substrate 90 slightly spaced from lines 94 and 95, having an identical resonance frequency, and coupling beam 93 that couples the two resonators.
A signal fed into input line 94 generates electric field responsive to the frequency of the signal and applies electrostatic force to resonator 91. At this time, when the frequency of the input signal generally agrees with the resonance frequency of resonator 91, resonator 91 is excited to vibrate, and resonator 92 coupled to resonator 91 with beam 93 also vibrates.
As such, only a signal having a frequency generally agreeing with the resonance frequency of resonators 91, 92 is selectively converted from an electric signal to mechanical vibrations. Then the mechanical signal is converted again to an electric signal between resonator 92 and output line 95. This is an inverse conversion to the conversion from the electric signal to the mechanical signal done between input line 94 and resonator 92.
The foregoing structure can work as a filter such that among signals fed into the input line, only the signals having a frequency generally agreeing with the resonance frequency of resonator 91, 92 are allowed to pass through output line 95. Resonance frequency xe2x80x9cf0xe2x80x9d of resonator 91 is expressed with the equation below:       f    0    =            1              2        ⁢                  xe2x80x83                ⁢        π              ⁢                  k        m            
where resonance frequency f0 is a function of mass xe2x80x9cmxe2x80x9d of resonator 91 and spring constant xe2x80x9ckxe2x80x9d. A similar equation is applicable to resonator 92.
Another conventional filter is disclosed in Japanese Patent Application Non-examined Publication No. H05-327393. This filter receives an unprocessed signal at an excitation coil, and oscillatory-wave components, of which frequency generally agrees with the resonance frequency of the resonators, are extracted out of the oscillatory waves of the unprocessed signal. This extraction is carried out by launching light from a fixed scale to a variable scale disposed at an oscillator, and changes of the power of the reflected light is extracted. As a result, this filter allows only the frequency resonant with the oscillator to pass through.
In order to work the conventional filters discussed above at frequencies of MHz band or GHz band, the mass of the resonators should be micro-miniaturized, which naturally requires the filter per se to be downsized to a micro-body.
For instance, FIG. 12 shows relations between resonance frequencies and lengths of resonators in the case of scaling down resonators 91, 92 of the conventional filter shown in FIG. 11. Resonators 91, 92 are actually 40 xcexcm long and 3 xcexcm wide, and those dimensions are scaled down with the same ratio.
In order to use this conventional filter as a device in the mobile communication field where a frequency band ranging of 1 GHz-5 GHz holds great promise to use this kind of filters, the length should be shortened to 0.04 xcexcm from 0.2 xcexcm. The relative distance between input line 94 and output line 95 placed via resonators 91, 92 is naturally required to be shorter.
As a result, in the conventional filter, input line 94 is placed closer to output line 95, and they make a direct coupling between them, so that the isolation lowers and the filter does not work properly.
FIG. 13A shows isolation characteristics of a filter having no direct coupling between an input line and an output line. FIG. 13B shows isolation characteristics of a filter where a coupling of 0.1 xcexcm space between an input signal and an output line is produced. In the case where the frequency is so low that a width between input line 94 and the output line 95 can be prepared wide enough to neglect a coupling between the two lines, the filter can work properly as shown in FIG. 13A. However, as the available frequency becomes higher, the resonator becomes smaller, and when input line 94 is directly coupled to output line 95, isolation in the frequencies higher than the resonance frequency greatly lowers as shown in FIG. 13B. As a result, the filter cannot work properly. On the other hand, in the frequencies lower than the resonance frequency, a capacitance generated between input line 94 and output line 95 resonates with an inductance component of the resonator, thereby sometimes producing unnecessary notches.
The filter in which input line 94 is placed close to output line 95 can be downsized to a micro-body process-wise; in fact, a direct coupling between the two lines degrades the filter characteristics, and the filter thus cannot be used in high frequencies such as MHz band or GHz band.
A filter used in high frequencies such as MHz band or GHz band includes resonators of micro-body of xcexcm order, so that its oscillators (resonators) are hard to oscillate properly due to the viscosity of air.
The present invention addresses the problems discussed above and aims to provide a filter free from characteristics degradation due to a direct coupling between an input line and an output line in high frequencies such as MHz band or GHz band. Further the filter of the present invention includes a resonator not influenced by the viscosity of air.
The filter of the present invention comprises the following elements:
a line through which an electric signal is input;
a resonator, disposed closely to the line and in vacuum, for resonating by applying electrostatic force of electric field generated responsive to a frequency of the electric signal; and
detecting means for detecting mechanical vibrations of the resonator.
The detecting means detects mechanical vibrations as a signal in another form than the electric signal.
Since the input electric signal is output in another form, this structure does not permit an input electric signal to be directly coupled to an output side. Even if an input side is placed immediately close to an output side because the resonator is downsized to a micro-body in high frequencies such as MHz band or GHz band. Further the resonator works in the vacuum, the resonator of a micro-body is not influenced by the viscosity of air, and micro-mechanical vibrations of the resonator can be converted into an appropriate signal before being detected. The vacuum referred in the present invention includes a true vacuum condition and a substantially vacuum condition not more than 100 pascal.